The author of the (highly recommended!) Tumblr blog The Unit of Caring responds to an anonymous correspondent's observation that trans-exclusionary radical feminists tend to define the word woman as "adult human biological female":

Oh, yeah, sorry, I've heard that one too though I've yet to find anyone willing to justify it. If you can find anyone explaining why this is a good definition, or even explaining what good properties it has, I'd appreciate it because I did sincerely put in the effort and—uncharitably, it’s as if there’s just 'matches historical use' and 'doesn’t involve any people I consider icky being in my category'.

I'm happy to try to help if I can!

I would say that a notable good property of the "adult human female" definition is non-circularity: we can articulate membership tests that do a pretty good job of narrowing down which entities do and do not belong to the category we're trying to talk about, without appealing to the category itself. Does the person have a vagina, ovaries, breasts, and two X chromosomes? That's a woman. Has the person given birth? Definitely a woman. Does the person have a penis? Definitely not a woman. This at least gives us a starting point from which we can begin to use this woman concept to make sense of the world, even if it's not immediately clear whether and how we should apply it to various comparatively rare edge-cases. (What about female-to-male transsexuals, a.k.a. trans men? What about people with complete androgen insensitivity syndrome? &c.)

In contrast, a strict gender-identity-based definition doesn't have this useful non-circularity property. If all I know about women is that women are defined as people who identify as women, I can't use that definition to figure out which people are women. This point may be more apparent if you substitute some completely foreign concept for women. If someone told you that zorplebobben are people who identify as zorplebobben, you would probably have questions about what that means! Why do they identify as zorplebobben? Given that someone is a zorplebobben, what else should I infer about them? The self-identity criterion doesn't help: without a base case, the infinite recursion of (people who identify as (people who identify as (people who identify as ...))) never terminates.

Of course, people who believe in the primacy of gender identity aren't trying to engage in circular reasoning. If they are making a philosophical mistake, there has to be some explanation of what makes the mistake appealing enough for so many people to make it.

But it's not hard to guess: there are, empirically, a small-but-not-vanishingly-small minority of people with a penis, XY chromosomes, facial hair, &c. who wish that they had a vagina, XX chromosomes, breasts, &c., and in a enlightened technological civilization, it seems humane to accommodate this desire as much as feasible, by giving people access to hormones and surgeries that approximate the phenotype of the other sex, respecting their chosen pronouns, &c. Thus we can legitimately end up with a non-circular trans-inclusive sense of the word women: "adult human females, and also adult human males who have undergone interventions to resemble adult human females sufficiently closely so that they can be taken as such socially."

But this is a mere broadening of the "adult human female" definition that tacks on extra complexity (partially for humanitarian reasons and partially to better predict social phenomena that most people care more about modeling well than biological minutiæ). The core idea is still intact and centered, such that even if we end up using the disjunctive, trans-inclusive sense a lot of the time, the narrower, trans-exclusive sense is still pretty salient, rather than being a perplexingly unmotivated notion with no good properties.

One might counterargue that this is unjustifiably assuming "biologically female" as a primitive. The author seems to endorse a critique along these lines the first of three objections to the "adult human female" criterion of womanhood—

1) The way we draw categories in biology is a social decision we make for social and cultural reasons, it isn’t a feature of the biology itself. A different sort of society might categorize infertile humans as a separate gender, for example, and that'd be as justified by the biology as our system. Or have 'prepubescent' be a gender, or 'having living offspring' be a gender—there are a million things that these categories could just as reasonably, from the biology, have been drawn around.

I've addressed this class of argument at length (about 7500 words) in a previous post, "The Categories Were Made for Man to Make Predictions", but to summarize briefly, while I agree that categories can be defined in many ways to suit different cultural priorities, it's also the case that not all possible categories are equally useful, because the cognitive function of categories is to group similar things together so that we can make similar predictions about them, and not every possible grouping of entities yields a "tight" distribution of predictions that can be usefully abstracted over.

A free-thinking biologist certainly could choose to reject the orthodoxy of grouping living things by ancestry and reproductive isolation and instead choose to study living things that are yellow, but their treatises would probably be difficult to follow, because "living things that are yellow" is intrinsically a much less cohesive subject matter than, say, "birds": experience with black crows is probably going to be more useful when studying yellow canaries than experience with yellow daffodils—even if, in all philosophical strictness, there are a million things that these categories could have been drawn around, and who can say but that some hypothetical other culture might have chosen color rather than ancestry as the true determinant of "species"?

It is of course true that different cultures will place different emphases and interpretations on various ways in which people can differ: being prepubescent or being a parent might have special significance in some cultures that outsiders could never understand. But to say that prepubescents might as well be a "gender"—well, at this point I must confess that I'm really not sure what this "gender" thing is that the author is trying to talk about.

And I guess that's the problem. People who assume a TERFy definition of woman—like, say, the authors of the Merriam–Webster dictionary ("noun, 1.a., an adult female person")—generally aren't trying to invalidate anyone's "gender"; they're trying to talk about biological sex using simple, universally-understood words. Biological sex is obviously not the only category in the world: in a lot of situations, you might care more about whether someone has living children—or for that matter, whether an organism is yellow—than what sex it is.

But when people do want to talk about sex—when they want to carve reality along that particular joint, without denying that there are superexponentially many others in the vastness of configuration space—there's something profoundly frustrating about Blue Tribe culture's axiomatic insistence that certain inferences must not be made, that certain conceptual distinctions must not be expressible, except perhaps cloaked behind polysyllabic obfuscations like "assigned sex at birth" (as if the doctors made a mistake!).

Even if many usages of words like woman can and should be interpreted in a trans-inclusive sense, it's important that it also be possible to sometimes use the words in a trans-exclusive sense in those cases where the distributions of trans people and cis people of a given "gender" differ significantly for the variables of interest. The point is not to be mean to trans women (who are a huge fraction of my and The Unit of Caring author's friends); the point is that it should be socially acceptable to describe reality using words.

Bob: I don't see the problem. It's a girl's track meet. Trans girls are girls, by definition. Why shouldn't they be allowed to compete with other girls?

Alice: ...

⁕ ⁕ ⁕

Alice: I'm sad that the sex ratio of my local decision-theory and compiler-development unified meetup group is so horribly lopsided, because this observation is in tension with my beautiful and sacred moral ideal of neither sex having a monopoly on any kind of virtue! If there's anything my native subcultures are doing to needlessly antagonize women, then that's wrong and I want to fix it!

Bob: What are you talking about? There were lots of women at that meetup.

Alice: I mean, yes, but literally all of us were trans.

Bob: So?

Alice: ...

⁕ ⁕ ⁕

Alice: Have you seen Dhejne et al.'s long-term followup study of transsexuals in Sweden? In Tables S1 and S2, the authors report that trans women committed violent crimes at far higher rates than cis women, with an adjusted-for-immigrant-and-psychiatric-status hazard ratio of 18.1—but only slightly lower rates than cis men, against whom the adjusted hazard ratio was 0.8.

Bob: Yes, how terrible that we still live in such a transphobic Society that those poor marginalized trans women are disproportionately driven to violent crime!

Alice: That's one theory. Can you think of any other possible interpretations of the data?

Bob: No.

Alice: Like, what do you make of the observation that the trans women's violent crime rate was not just higher than cis women's, but also strikingly close to that of cis men? Can you think of any reason—any reason at all—why that might not be a coincidence?

Bob: No, that has to be a coincidence. What could trans women and cis men possibly have in common?

Alice: ...

⁕ ⁕ ⁕

(Another dialogue about reproduction belongs in this collection, but was deemed too obvious and has been cut for space.)

⁕ ⁕ ⁕

The point being illustrated here is that if it's socially unacceptable for people who want to talk about sex to say "That's not what I meant by woman in this context and you know it", then people who would prefer not to acknowledge sex will always get the last word, not because they have superior arguments, but because the very terms of discourse have been systematically gamed to conflate dissent with unkindness.

To this it might be objected that trans activists and allies are merely advocating for greater precision, rather than trying to make it socially unacceptable to think about biological sex: after all, you can just say "cis women" (which excludes trans women, trans men, and natal-female nonbinary people) or "assigned female at birth" (which excludes trans women, but includes trans men and natal-female nonbinary people and presumably David Reimer) or "people with uteruses" (which excludes trans women and natal females who have had a hysterectomy) if that's what you really mean.

Alternatively, we could imagine people agreeing that word woman refers solely to social roles and personal identity and must always be used in a trans-inclusive sense, while reserving female for when people want to talk about biological sex. However, I get the sense that this is not a compromise most contemporary trans activists would find acceptable: witness, for example, Zinnia Jones proclaiming that "[t]rans women are female—with female penises, female prostates, female sperm, and female XY chromosomes." (!)

Ultimately, I think all this is underestimating the usefulness of having simple, short descriptions for the categories that do the most predictive work on typical cases.

Kind or not, morally justified or not, voluntary or not, sexual dimorphism is actually a real thing. Studying the pages of Gray's Anatomy—or Wikipedia if you're on a budget—you can absorb all sorts of detailed, specific knowledge of the differences between female and male humans, from the obvious (sex organs, vocal pitch, height, muscle mass, body hair) to the less-obvious-but-well-known (chromosomes, hormones, pelvis shape) to the comparatively obscure (blood pressure! lymphocyte concentrations! gray-matter-to-white-matter ratios in the brain!). Nor is this surprising from a theoretical standpoint, where we have theories explaining mechanismsby which sexual dimorphism can evolve and what kinds of differences it produces in different species.

If—like me—you're the kind of person who is not necessarily happy about sexual dimorphism, you can always deliberately define your categories in order to minimize it: if there's a large sex difference in some observable measurement, just say you don't care about predicting that particular measurement.

But people who have other concerns than minimizing Blue Tribe people's quasi-religious discomfort with sexual dimorphism (it's my former quasi-religion, too, so I'm allowed to make fun of us) might want a common word—or even just a particular sense of a common word—to describe the world they see, in which sex is a real thing worth noticing.

It might be worth noticing even if you don't believe in psychological sex differences! That's why generations of feminists have fought valiantly for women's rights on the grounds that women are every bit the moral and intellectual equals of men, rather than the grounds that it's not clear whether "women" actually exist as a non-arbitrary category.

Being limited to just saying "people with uteruses" when the topic of conversation happens to be childbearing (or whatever the approved socially-just construction turns out to be) is not a suitable replacement (per Alicorn's maxim) when the speaker wants to refer to all the other dimensions along which women statistically have things in common, including things that are hard to articulate or measure.

And including things that may not even be currently known. I certainly don't know what differences in gray-to-white brain matter ratios mean psychologically, but my map is not the territory: it doesn't mean some future sufficiently-advanced neuroscience won't be able to say what the difference means about female and male minds, and some sufficiently advanced evolutionary psychology, under what selection pressures it evolved.

Speaking of future advances in knowledge, the author continues to her second objection—

2) Someday people are just going to be able to generate the exact physical body they want to inhabit. At that point, "biological" anything isn't going to apply.

I definitely agree that biological anything isn't going to apply in the glorious posthuman future of unimaginable power and freedom when people can reshape their body and mind at will.

But it's also not clear how much relevance this science-fictional scenario has to people in the unglorious preposthuman present. Yes, we do have HRT and SRS, and these are magnificent achievements for the grand cause of morphological freedom, and should be available on an informed-consent basis. It's definitely something.

But it's also definitely not-everything. To get a sense of how far we have to go, I strongly recommend reading Eliezer Yudkowsky's heartbreaking 2009 take on what an actually effective male-to-female sex change would take.

In my youth, I used to be more optimistic about the future of human enhancement. "Oh, sure, that may be true of present-day humans, but in general ..." felt like a relevant and useful form of argument.

These days, dwelling on the general case feels awfully pedantic. I think what changed is that as I read more and gained some personal experience with real-world technology development (albeit in mere software), I began to appreciate technology as the sum of many contingent developments with particular implementation details that someone had to spend thousands of engineer–years pinning down, rather than as an unspecified generic force of everything getting better over time. In principle, everything not directly prohibited by the laws of physics is probably possible, which basically amounts to any miracle you can imagine. In practice, we get a very few, very specific miracles that depend on vast institutions and supply chains and knowledge that can be lost as well as gained.

Alice: When I lost my mother, I knew I could not rest until Death itself is defeated!

Bob: But as long as you remember her, your mother lives on in you!

Alice: I mean, metaphorically yes, but I meant deathas in, like, the cessation of all biological functions that sustain a living organism.

Bob: Oh, yeah, sorry, I've heard that one, too, though I've yet to find anyone willing to justify it. If you can find anyone explaining why this is a good definition, or even explaining what good properties it has, I'd appreciate it, because I did sincerely put in the effort and—uncharitably, it's as if there's just 'matches historical use' and 'doesn't involve icky people from the past being in my category'.

Alice: ...

⁕ ⁕ ⁕

The Unit of Caring author continues:

If your definition of a 'woman' is one where trans people will be their preferred gender once the tech catches up, then I think you should probably reflect on what actually changes about anyone's lived experience on that magic day when our cyborgs hit your threshold. And if it isn't, then you're stuck asserting that if a woman is cell-for-cell identical to me then she still might not be a 'biological woman'. That's a sign that this isn't actually about biology.

I would rather say that's a sign that we're facing an instance of the Sorites paradox, the ancient challenge to applying discrete categories to a continuous world. If one grain of sand doesn't make a heap (the argument goes), and the addition of one more grain of sand can't change whether something is a heap, then we can conclude from the principle of mathematical induction that no number n ∈ ℕ of grains make a heap. (Or, alternatively, that the absence of any sand constitutes a "heap of zero grains".) Analogously, if a sufficiently small change in MtF transition outcome can't change whether someone is a woman, then we are seemingly forced to accept that either everyone is a woman or no one is.

While the Sorites paradox is certainly an instructive exercise in the philosophy of language, its practical impact seems limited: most people find it more palatable to conclude that that the heap-ness is a somewhat fuzzy concept, rather than to concede that the argument isn't actually about the amount of sand in a location. And if you brought a single grain of sand when someone asked you for a heap, they probably wouldn't hesitate to say, "That's not what I meant by heap in this context and you know it."

If that's the side of this question you come down on, then I encourage you to ask yourself why that trans women still doesn't count. I expect that whatever your answer, that's the real definition you’re using, not "biological".

I definitely agree that this is a valuable thought experiment: in this limit of perfect physical transition technology, what possible reasons could there be to deny that trans women are women? Allow me to give a conditional answer.

If psychological sex differences aren't real, then there aren't any: ex hypothesi, the physiological differences between females and males are the only thing for the word woman to attach to, and ex hypothesi, we know how to fix those.

Alternatively, if psychological sex differences are a thing, and transness is a brain intersex condition such that pre-transition trans women are already psychologically female, then again, there aren't any: ex hypothesi&c.

However, if we should be so unlucky to live in a world in which psychological sex differences are a thing and most trans women are motivated to transition by some other reason than already having female minds, then we face some subtleties: if our thought-experimental perfect transition tech doesn't edit minds, then we end up with a bunch of female-bodied people with a distribution of psychologies that isn't just not-identical to that of natal females, but is actually coming out of the male distribution. Should such people be called women? Honestly, I lean towards Yes, but I can at least see the argument of someone who preferred not to use the word that way.

Wrapping up—

3) What does this definition of 'woman' get you?

It gets us a concept to refer to the set of adult human females. (Even if, again, we often also use the word woman in a broader trans-inclusive sense; it's not uncommon for words to have both narrower and broader definitions which can be distinguished from context.)

If the concept of women in the narrow, trans-exclusionary sense is to be forbidden from polite Society, then people trying to make sense of their experiences will be forced to reinvent it, probably by means of obfuscatory neologisms ("assigned female at birth") coupled with the quietly indefatigable wordless anticipation that it's somehow not a coincidence that cis women and trans men and a.f.a.b. nonbinary people get pregnant sometimes, but cis men and trans women and a.m.a.b. nonbinary people never do.

I want to live in that glorious future of Total Morphological Freedom. But nature to be commanded must be obeyed. To get godlike mastery over our physical forms, to break free of the prison of today's unremediated genderspace, is going to require a detailed understanding of exactly how things work today, as it is only from such knowledge that pallative interventions can be designed. And, bluntly, the fact that the smartest people I know tend to direct more of their effort towards redefining top-20 nouns than on biotechnology research, does not exactly inspire confidence or hope.